RegExp in ECMAScript 262

Object representing a regular expression.

A new RegExp object can be created via three methods::

var foo = /pattern/flags; //regexp literal
var foo = new RegExp( patternAsString , flags ); //constructing from a string and flags
var foo = new RegExp( existingRegExp ); //duplicating the existing regular expression

The regexp literal notation (first method above) is usually faster than the latter, and should be used when possible. The pattern for the regular expression, and the flags (zero or more of the characters g, i and m in any order) exist as literal text on the page with no other delimiters:

var oneCat = /cat/; //matches a single occurrence of "cat", case-sensitive
var anyCat = /cat/i; //matches a single occurence of "cat", case-insensitive
var all_cats = /cat/g; //matches every occurence of "cat", case-sensitive
var allCats = /cat/gi; //matches every occurence of "cat", case-insensitive

var oneInteger = /\d+/; //matches a single occurence one or more digits, 0-9
var allIntegers = /\d+/g; //matches every occurence of one or more digits

var theFirstLine = /^.+/g; //matches from the start of the string to the first newline
var everyLine = /^.+/gm; //matches from the start of a new line to end of that line

When the new RegExp(pattern, flags) constructor is used, pattern and flags are both strings. (Though flags may be omitted.)

var oneCat = new RegExp("cat");
var allCats = new RegExp("cat","gi");

The new RegExp() constructor should primarily be used only when the full regular expression is not known during creation, but must be constructed from a string.

function KillHTMLTag(sourceString,htmlTagName,caseSensitive){
    var flags = "g";
    if (!caseSensitive) flags+="i";
    var killRE = new RegExp("</?"+htmlTagName+"[^>]*>",flags);
    return sourceString.replace(killRE,'');
}
var html = "<p>Hello <b>World</b></p>";
html = KillHTMLTag(html,'B');
// ** html is now "<p>Hello World</p>"

When using the new RegExp() constructor to create a regular expression from a string, remember that both string escaping and special regular expression escaping are needed:

var oneInteger = /\d+/;
var oneInteger = new RegExp("\\d+");

var findDogBackslashCat = /Dog\\Cat/;
var findDogBackslashCat = new RegExp("Dog\\\\Cat"); //Extra \ needed in string form

A full discussion of regular expression syntax is beyond the scope of this reference. The MSDN Introduction to Regular Expressions may be helpful to the beginner, especially the Regular Expression Syntax section.